In a variety of wellbore applications, electrical signals are sent between a surface location and a downhole location. The transmission of electricity within the wellbore enables powering of downhole components, downhole data acquisition, activation and control of downhole devices, and numerous other applications. For example, command and control signals may be sent from a controller located at the surface to a wellbore device located within a wellbore. In other applications, downhole devices, such as downhole gauge systems, collect data and relay that data to a surface location through an “uplink” for evaluation or use in the specific well related operation.
The transmission of electricity relies on conductive wires or electrical cables to conduct electrical signals between downhole and uphole devices. However, the conductive wires or electrical cables contribute added expense to wellbore systems. Additionally, the wires and/or cables can complicate installation of the downhole system and create reliability problems. For example, wire and cable connectors are susceptible to damage and degradation due to the often harsh wellbore environment. Thus, wires and/or cables always carry a risk of breakage and often must be anchored to other well system components by additional components, such as cable protectors. These are just some of the examples of difficulties that can arise with the use of wires and/or cables for carrying electrical signals in a wellbore environment.
Attempts also have been made to utilize existing wellbore system structure in mimicking a coaxial cable. For example, a coaxial cable structure has been constructed using the production tubing and well casing as conductive electric signal carriers. A substantially non-conducting fluid, such as diesel, is required in the annulus between the production tubing and the well casing. The use of such fluid, however, is not a typical completion practice, and the non-connecting fluid annulus must be maintained for the telemetry to function.